This invention relates to the method of mechano-chemical polishing crystals and epitaxial layers of GaN and GaAlInN in order to remove of irregularities and highly defected regions from the surface of crystals and epitaxial layers of GaN and Ga1xe2x88x92xxe2x88x92yAlxInyN used in manufacturing of optoelectronic devices.
There are well known methods of preparation of the surface of substrates of single GaN crystals for homoepitaxy based on polishing by diamond micropowder and high temperature annealing in the atmosphere of ammonia and hydrogen gas mixture. The micropowder polishing leads creation of the surface with ridges of the height of several atomic layers with the of the depth of order of 200 A i.e. about 100 interatomic distances. The annealing in ammonia and hydrogen gas mixture do not remove the scratches despite some change of the surface structure.
The flaw of diamond micropowder polishing and annealing method is creation of highly defected surface which leads to important disturbances of the growth of epitaxial layers.
There are also well known methods of removal of surface irregularities of other crystals by polishing on the soft pads in presence of chemical etching substances not being the water solution of the concentration higher than 0.01 so called mechano-chemical polishing. The mechano-chemical polishing process proceeds via creation of a thin layer of soft reaction product which is removed by soft pad polishing.
One object of the Invention is the method of removal of surface irregularities and highly defected regions from the surface of crystals and epitaxial layers of GaAlInN by polishing on soft pad in presence of base water solution of the concentration above 0.1 during 10 second and subsequent replacement of the solution by pure water and polishing by at least 1 minute. Subsequently the pressure exerted on the sample is diminished and the polishing machine is stopped and the polished GaAlInN crystal is removed in a well known method under the flow of dry nitrogen. As a chemical etching agent the water solutions of the bases such as sodium base NaOH, potassium base KOH or their mixtures. The polishing is effected on a soft pad in presence of etching agent and water under the pressure of 0.01 MPa and using angular velocity of 1 to 100 revolutions per minute. The polishing in the presence of etching agent is effected with the continues adding of etching agent with the rate of 1-5 droplets per second. The water polishing is effected with continues adding the pure water with the rate higher than 1 droplet for second. The advantage of the Invention is a possibility of yielding of atomically smooth large area surfaces of crystals and epitaxial layers of GaAlInN. The crystals with so prepared surfaces are the optimal substrates for homoepitaxy of thin layers and highest quality electronic structures. The method of the Invention will not lead to creation of flat layer of the reaction product but the etching ot the surface into the sharp edged pyramidal structure. The polishing according to the Invention will lead to abrasion of the pyramids created in the etching process because local strain on the sharp edges considerably overcomes the nominal value of the pressure. The method of the Invention is simple and can be used in room temperature. The chemical etching agents are easy to prepare, and nontoxic.